3. Python

"Copy"

Python (comments) hash # (string literal) hash hash Python

# 
spam = 1  # 
          # ... 
text = "# "

3.1. Python

Python >>>

3.1.1. (Number)

expressionExpression +-* / ()

>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # 
1.6

2420 int 5.01.6 float

(/) float floor division // %

>>> 17 / 3  # 
5.666666666666667
>>>
>>> 17 // 3  # 
5
>>> 17 % 3  # % 
2
>>> 5 * 3 + 2  #  *  + 
17

Python (powers) ** [1]

>>> 5 ** 2  # 5 
25
>>> 2 ** 7  # 2  7 
128

(=)

>>> width = 20
>>> height = 5 * 9
>>> width * height
900

(defined)

>>> n  # 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

(operator) (operand)

>>> 4 * 3.75 - 1
14.0

_ Python

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

---

int floatPython Decimal FractionPython (complex numbers) j J 3+5j

3.1.2.

Python str (strings) "!" "rabbit" "Paris" "Got your back." "Yay! :)" ('...') ("...") [2]

>>> 'spam eggs'  # 
'spam eggs'
>>> "Paris rabbit got your back :)! Yay!"  # 
'Paris rabbit got your back :)! Yay!'
>>> '1975'  # 
'1975'

\

>>> 'doesn\'t'  #  \' ...
"doesn't"
>>> "doesn't"  # ...
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'

Python shell print()

>>> s = 'First line.\nSecond line.'  # \n 
>>> s  #  print()
'First line.\nSecond line.'
>>> print(s)  #  print() \n 
First line.
Second line.

\ raw string r

>>> print('C:\this\name')  #  \t  tab\n 
C:      his
ame
>>> print(r'C:\this\name')  #  r
C:\this\name

\

"""...""" '''...''' \

>>> print("""\
... Usage: thingy [OPTIONS]
...      -h                        Display this usage message
...      -H hostname               Hostname to connect to
... """)
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to

>>>

+ (concatenate) *

>>> # 3  'un' 'ium'
>>> 3 * 'un' + 'ium'
'unununium'

string literal

>>> 'Py' 'thon'
'Python'

>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'

>>> prefix = 'Py'
>>> prefix 'thon'  # 
  File "<stdin>", line 1
    prefix 'thon'
           ^^^^^^
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
  File "<stdin>", line 1
    ('un' * 3) 'ium'
               ^^^^^
SyntaxError: invalid syntax

+

>>> prefix + 'thon'
'Python'

indexed( subscripted) 0 1

>>> word = 'Python'
>>> word[0]  #  0 
'P'
>>> word[5]  #  5 
'n'

>>> word[-1]  # 
'n'
>>> word[-2]  # 
'o'
>>> word[-6]
'P'

-0 0 -1

slicing (substring)

>>> word[0:2]  #  0 ()  2 () 
'Py'
>>> word[2:5]  #  2 ()  5 () 
'tho'

(slice indices) 0 slice

>>> word[:2]   #  2 () 
'Py'
>>> word[4:]   #  4 () 
'on'
>>> word[-2:]  #  () 
'on'

s[:i] + s[i:] s

>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

slice slice 0 n n

 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1

0...6 i j slice i j

slice word[1:3] 2

>>> word[42]  # 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

slice

>>> word[4:42]
'on'
>>> word[42:]
''

Python --- immutable

>>> word[0] = 'J'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'

len()

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34

Text Sequence Type --- str: sequence
: method
f-stringf
: str.format() (string formatting)
printf-style String Formatting: %

3.1.3. List

Python list itemList

>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]

sequence list (slice)

>>> squares[0]  # 
1
>>> squares[-1]
25
>>> squares[-3:]  # 
[9, 16, 25]

List (concatenation)

>>> squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

immutablelist mutable list

>>> cubes = [1, 8, 27, 65, 125]  # 
>>> 4 ** 3  # 4  64 65
64
>>> cubes[3] = 64  # 
>>> cubes
[1, 8, 27, 64, 125]

list list.append()

>>> cubes.append(216)  #  6 
>>> cubes.append(7 ** 3)  #  7 
>>> cubes
[1, 8, 27, 64, 125, 216, 343]

Python list list list

>>> rgb = ["Red", "Green", "Blue"]
>>> rgba = rgb
>>> id(rgb) == id(rgba)  # 
True
>>> rgba.append("Alph")
>>> rgb
["Red", "Green", "Blue", "Alph"]

list list

>>> correct_rgba = rgba[:]
>>> correct_rgba[-1] = "Alpha"
>>> correct_rgba
["Red", "Green", "Blue", "Alpha"]
>>> rgba
["Red", "Green", "Blue", "Alph"]

slice list list

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> letters
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> # replace some values
>>> letters[2:5] = ['C', 'D', 'E']
>>> letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>> # now remove them
>>> letters[2:5] = []
>>> letters
['a', 'b', 'f', 'g']
>>> # 
>>> letters[:] = []
>>> letters
[]

len() list

>>> letters = ['a', 'b', 'c', 'd']
>>> len(letters)
4

list list list

>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

3.2.

Python 2 2

>>> # 
>>> # 
>>> a, b = 0, 1
>>> while a < 10:
...     print(a)
...     a, b = b, a+b
...
0
1
1
2
3
5
8

a b 0 1 (expression) (evaluate)
while a < 10 Python C (true) (false)list (comparison operators) C <>==<=>= !=
Python tab

print()

>>> i = 256*256
>>> print('The value of i is', i)
The value of i is 65536

end

>>> a, b = 0, 1
>>> while a < 1000:
...     print(a, end=',')
...     a, b = b, a+b
...
0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

[1]

** - -3**2 -(3**2) -9 9 (-3)**2

[2]

\n ('...') ("...") " \'